Methods of fabricating multi-layer nonvolatile memory devices

ABSTRACT

A nonvolatile memory device includes a semiconductor substrate having a first well region of a first conductivity type, and at least one semiconductor layer formed on the semiconductor substrate. A first cell array is formed on the semiconductor substrate, and a second cell array formed on the semiconductor layer. The semiconductor layer includes a second well region of the first conductivity type having a doping concentration greater than a doping concentration of the first well region of the first conductivity type. As the doping concentration of the second well region is increased, a resistance difference may be reduced between the first and second well regions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of U.S. patentapplication Ser. No. 11/654,133, filed in the United States PatentOffice on Jan. 17, 2007, and claims priority under 35 U.S.C. § 119 toKorean Patent Application No. 2006-103050, filed on Oct. 23, 2006, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention disclosed herein relates to semiconductor devicesand methods of fabricating the same, and, more particularly, tomulti-layer nonvolatile memory devices and methods of fabricating thesame.

Semiconductor devices have, generally, reached the limit of reducing ahorizontal size thereof, and, thus, a three-dimensional structure with aplurality of cell array layers stacked has been researched to overcomesuch a limit.

To form a cell array layer in a multi-layer, a plurality ofsemiconductor layers may be formed on a semiconductor substrate.However, there may be a limit in a thickness of a semiconductor layercompared to a thickness of a semiconductor substrate, and, therefore, adevice formed on a semiconductor substrate may be different from adevice formed on a semiconductor layer in the uniformity of theirproperty.

FIG. 1 is a cross-sectional view of a conventional multi-layernonvolatile memory device.

Referring to FIG. 1, a nonvolatile memory device may be formed in a NANDcell array structure. The NAND cell array structure includes a groundselect line GSL and a string select line SSL that cross over a pluralityof parallel active regions, and a plurality of word lines WLn. The wordlines are disposed between the ground select line GSL and the stringselect line SSL to cross over the active regions.

The ground select line GSL, the string select line SSL, and the wordlines WLn are disposed mirror-symmetrically in a cell array.

An n-well region 12 and a p-well region 14 surrounded by the n-wellregion 12 are formed in a semiconductor substrate 10. A plurality ofp-well regions 14 may be formed in a first cell array region, and thep-well regions 14 may be isolated by the n-well region 12.

The ground select line GSL and the string select line SSL may include aselect gate 18 s formed on a gate insulating layer 16, and a maskpattern 20 s may be formed on the select gate 18 s. The word line WLnmay include a gate electrode 18 w formed on the gate insulating layer 16and a mask pattern 20 w formed on the gate electrode 18 w. The gateinsulating layer 16 may be a tunnel insulating layer in a floating gatetype nonvolatile memory device, or may be a multi-layered charge traplayer in a charge trap type nonvolatile memory device.

At least one semiconductor layer 26 may be formed over the semiconductorsubstrate 10, and a second cell array region may be formed on thesemiconductor layer 26. The second cell array region may have the samedisposition structure as the cell array region formed on thesemiconductor substrate 10. For example, the ground select line GSL andthe string select line SSL may include a select gate 30 s formed on agate insulating layer 28, and mask pattern 32 s may be formed on theselect gate 30 s. The word line WLn may include a gate electrode 30 wformed on the gate insulating layer 28 and a mask pattern 32 w formed onthe gate electrode 30 w. The gate insulating layer 28 may be a tunnelinsulating layer in a floating gate type nonvolatile memory device, or amulti-layer charge trap layer in a charge trap type nonvolatile memorydevice.

An interlayer insulating layer 22 is interposed between thesemiconductor layer 26 and the semiconductor substrate 10. Although notshown, an interlayer insulating layer is interposed between thesemiconductor layer 26 and another semiconductor layer. Thesemiconductor layer 26 may be crystal-grown from a semiconductor plug 24that penetrates the interlayer insulating layer 22 depending on themethod of forming the semiconductor layer 26. The semiconductor plug 24may be an epitaxial layer crystal-grown from another semiconductor layeror semiconductor substrate thereunder.

Generally, a cell array of a nonvolatile memory device may be dividedinto a plurality of erase blocks, and the erase blocks are divided intop-well regions isolated by an n-well region. The erase blocks may have apredetermined potential in an operating process. Therefore, it may bedesirable for a resistance of the p-well regions to be uniform for theuniformity of a bias applied to memory cells.

As illustrated, because the semiconductor layer 26 is formed over thesecond cell array region, its thickness may be limited due to theproperty degradation of the transistors formed thereunder. Therefore, awell region formed in the semiconductor layer 26 may have a limit inincreasing its depth, compared to the thick semiconductor substrate 10.Consequently, there may be a difference between a well region formed inthe semiconductor layer 26 and a well region formed in the semiconductorsubstrate 10. In addition, the well region formed in the semiconductorlayer 26 may have a relatively high electrical resistance, and, thus,dispersion of a memory cell transistor characteristics may increase.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide a multi-layernonvolatile memory device where there is little difference between thewell resistance of a semiconductor layer and the well resistance of asemiconductor substrate.

According to some embodiments of the present invention, a nonvolatilememory device includes a semiconductor layer that includes a well regionhaving a doping concentration that is higher than the dopingconcentration of a well region formed in a semiconductor substrate. Anonvolatile memory device, according to some embodiments of the presentinvention, includes a semiconductor substrate having a first well regionof a first conductivity type and at least one semiconductor layer formedon the semiconductor substrate. A first cell array is formed on thesemiconductor substrate, and a second cell array is formed on thesemiconductor layer. The semiconductor layer includes a second wellregion of a first conductivity type having a concentration greater thana concentration of the first well region of the first conductivity type.

In some embodiments, a resistance difference between the first andsecond well regions may be decreased by increasing a concentration ofthe second well region. For example, a surface doping concentration ofthe semiconductor layer may be greater than a well doping concentrationthereunder, and a surface doping concentration of the semiconductorlayer may be equal or similar to a surface doping concentration of thesemiconductor substrate. Therefore, a resistance difference between thesemiconductor substrate and the semiconductor layer may be reduced, anda difference in impurity concentration may be reduced between surfacesof the semiconductor substrate and the semiconductor layer where unitdevices are formed, thereby improving the property uniformity of thedevice.

In other embodiments of the present invention, nonvolatile memorydevices may be formed that include a semiconductor layer that includes awell region having a doping concentration greater than a dopingconcentration of a well region formed in a semiconductor substrate. Afirst well region is formed in a semiconductor substrate having a firstconductivity type, and an interlayer insulating layer is formed on thesemiconductor substrate. A semiconductor layer is formed on theinterlayer insulating layer. A second well region of the firstconductivity type is formed in the semiconductor layer, and a dopingconcentration of the second well region is greater than a dopingconcentration of the first well region.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of exemplary embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a conventional multi-layernonvolatile memory device;

FIG. 2 is a plan view of a conventional multi-layer nonvolatile memorydevice;

FIGS. 3A and 3B are cross-sectional views taken along lines I-I andII-II′ of FIG. 2, respectively, according to some embodiments of thepresent invention; and

FIGS. 4A through 7A are views taken along the line I-I′ of FIG. 2 forillustrating methods of fabricating a nonvolatile memory deviceaccording to some embodiments of the present invention;

FIGS. 4B through 7B are views taken along the line II-II′ of FIG. 2 forillustrating methods of fabricating a nonvolatile memory deviceaccording to some embodiments of the present invention;

FIGS. 8A through 10A are views taken along the line I-I′ of FIG. 2 forillustrating methods of fabricating a nonvolatile memory deviceaccording to further embodiments of the present invention;

FIGS. 8B through 10B are views taken along the line II-II′ of FIG. 2 forillustrating methods of fabricating a nonvolatile memory deviceaccording to further embodiment of the present invention;

FIG. 11 is a cross-sectional view for illustrating a well structureaccording to some embodiments of the present invention; and

FIGS. 12 and 13 are cross-sectional views for illustrating a transistorstructure according to some embodiments of the present invention.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likereference numerals refer to like elements throughout the description ofthe figures.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly on” another element, there are no intervening elementspresent. It will be understood that when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected or coupled” to another element, there are no interveningelements present. Furthermore, “connected” or “coupled” as used hereinmay include wirelessly connected or coupled. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first layer could be termed asecond layer, and, similarly, a second layer could be termed a firstlayer without departing from the teachings of the disclosure.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toother elements as illustrated in the Figures. It will be understood thatrelative terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the Figures. Forexample, if the device in one of the figures were turned over, elementsdescribed as being on the “lower” side of other elements would then beoriented on “upper” sides of the other elements. The exemplary term“lower”, can therefore, encompass both an orientation of “lower” and“upper,” depending of the particular orientation of the figure.Similarly, if the device in one of the figures is turned over, elementsdescribed as “below” or “beneath” other elements would then be oriented“above” the other elements. The exemplary terms “below” or “beneath”can, therefore, encompass both an orientation of above and below.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Embodiments of the present invention are described herein with referenceto cross section illustrations that are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, a region illustrated or described as flatmay, typically, have rough and/or nonlinear features. Moreover, sharpangles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present invention.

In the description, a term “substrate” used herein may include astructure based on a semiconductor, having a semiconductor surfaceexposed. It should be understood that such a structure may containsilicon, silicon on insulator, silicon on sapphire, doped or undopedsilicon, epitaxial layer supported by a semiconductor substrate, oranother structure of a semiconductor. And, the semiconductor may besilicon-germanium, germanium, or germanium arsenide, not limited tosilicon. In addition, the substrate described hereinafter may be one inwhich regions, conductive layers, insulation layers, their patterns,and/or junctions are formed.

FIG. 2 is a plan view of a conventional nonvolatile memory device havinga NAND type cell array.

Referring to FIG. 2, a device isolation layer is formed in asemiconductor substrate to define a plurality of active regions Act. Afirst cell array region includes a ground select line GSL and a stringselect line SSL that cross over the active regions, and a plurality ofword lines WLn. The word lines WLn are disposed between the groundselect line GSL and the string select line SSL to cross over the activeregions. The ground select line GSL, the string select line SSL, and theword lines WLn are disposed mirror-symmetrically in the first cellarray.

An information storage Cs is located at an intersecting portion betweenthe word line WLn and the active region Act. The information storage Csmay be a floating gate or a charge trap layer. A common source line GSLis disposed between the ground select lines GSL to electrically connectsource regions formed in the active regions Act thereunder. Bit linecontacts DC are connected to active regions between the string selectlines SSL, respectively.

FIGS. 3A and 3B are cross-sectional views taken along lines I-I′ andII-II′ of FIG. 2 for illustrating a nonvolatile memory device,respectively, according to some embodiments of the present invention.

Referring to FIGS. 3A and 3B, a first well region of a firstconductivity type 54 is formed in a semiconductor substrate 50, and thefirst well region 54 is surrounded by a second conductive well region52. The semiconductor substrate 50 may be a p-type substrate, the firstwell region 54 may be a p-type well region, and the second conductivitytype well region 52 may be an n-type well region. A plurality of p-wellregions 54 may be formed in the first cell array region, and the p-wellregions 54 may be isolated from each other by the n-well region 52.

The ground select line GSL and the string select line SSL include aselect gate 62 s formed on a gate insulating layer 60, a mask pattern 64s may be formed on the select gate 62 s. The word line WLn may include agate electrode 62 w formed on the gate insulating layer 60 and a maskpattern 64 w formed on the gate electrode 62 w. The gate insulatinglayer 60 may be a tunnel insulating layer in a floating gate typenonvolatile memory device, and may be a multi-layered charge trap layerin a charge trap type nonvolatile memory device.

At least one semiconductor layer may be formed over the semiconductorsubstrate 50. The semiconductor layer may include a first semiconductorlayer 70 and a second semiconductor layer 74 formed on the firstsemiconductor layer 70. A second cell array region may be disposed onthe second semiconductor layer 74. The second cell array may also havethe same disposition structure as the first cell array region formed onthe semiconductor substrate 50. For example, the ground select line GSLand the string select line SSL may include a select gate 82 s formed ona gate insulating layer 80, and a mask pattern 84 s may be formed on theselect gate 82 s. The word line WLn may include a gate electrode 82 wformed on the gate insulating layer 80 and a mask pattern 84 w formed onthe gate electrode 82 w. The gate insulating layer 80 may be a tunnelinsulating layer in a floating gate type nonvolatile memory device, andmay be a multi-layered charge trap layer in a charge trap typenonvolatile memory device. The second cell array may have a sourceregion 75 s and a drain region 75 d formed in the second semiconductorlayer 74.

An interlayer insulating layer 66 is interposed between thesemiconductor layer and the semiconductor substrate 50. Although notshown, an interlayer insulating layer may be interposed between thesemiconductor layer and another semiconductor layer. The semiconductorlayer may be crystal-grown from a semiconductor plug 68 that penetratesthe interlayer insulating layer 66 depending on the method of formingthe semiconductor layer. The first semiconductor plug 70 may be anepitaxial layer crystal-grown from another semiconductor layer orsemiconductor substrate formed thereunder.

In the first embodiment of the present invention, the semiconductorlayer may include a second well region of a first conductivity typehaving a doping concentration higher than that of the first well region54. The second well region of a first conductivity type may be formed inthe first semiconductor layer 70. That is, p-type impurities may beimplanted into the first semiconductor layer 70 to form the second wellregion of a first conductivity type. The second semiconductor layer 74may include a third well region of a first conductivity type having adoping concentration equal to or lower than that of the second wellregion. For example, the second semiconductor layer 74 may be doped withp-type impurities. It some embodiments, the doping concentration of thesecond semiconductor layer 74 is equal or similar to that of the firstwell region 54.

In some embodiments of the present invention, the first semiconductorlayer 70 may be thinner than the second semiconductor layer 74. A deviceisolation layer 56 may be disposed in the semiconductor substrate 50 todefine a plurality of active regions 58, and a device isolation layer 78may be disposed in the semiconductor layer to define a plurality ofactive regions. The device isolation layer 78 may be formed in thesecond semiconductor layer 74. The thickness of the second semiconductorlayer 74 may be controlled to correspond to the thickness of the deviceisolation layer 78. Therefore, the second semiconductor layer 74 mayhave a thickness that is sufficient to form a device isolation layerhaving a thickness suitable for device isolation.

The third well region is formed in the active region between the deviceisolation layers 78, and thereby its lateral border may be defined bythe device isolation layers 78. In some embodiments of the presentinvention, the second well region of the first semiconductor layer 70may be doped at a concentration higher than that of the first wellregion 54 so as to reduce an electrical resistance. The third wellregion may be doped at a concentration equal or similar to that of thefirst well region 54 so as to improve the property uniformity oftransistors formed on the semiconductor substrate 50.

In further embodiments, the second well region may be formed in thefirst semiconductor layer 70 such that it is aligned with the deviceisolation layers 78. That is, the second well region may be limitedlyformed in portions of the first semiconductor layer 70 under the secondsemiconductor layer 74 between the device isolation layers 78.

FIGS. 4A through 7A are sectional views taken along the line I-I′ ofFIG. 2 for illustrating methods of fabricating a nonvolatile memorydevice according to some embodiments of the present invention. FIGS. 4Bthrough 7B are sectional views taken along the line II-II′ of FIG. 2 forillustrating methods of fabricating a nonvolatile memory deviceaccording to some embodiments of the present invention.

Referring to FIGS. 4A and 4B, a second conductivity type well region 52is formed in a semiconductor substrate 50, and a first well region 54 ofa first conductivity type is formed on the second conductivity type wellregion 52. The first well region 54 is surrounded by the secondconductivity type well region 52. The semiconductor substrate 50 may bea p-type substrate, the first well region 54 may be a p-type wellregion, and the second conductivity type well 52 may be an n-type wellregion. A plurality of p-well regions 54 may be formed in a cell arrayregion, and the p-well regions 54 may be isolated from each other by then-well region 52.

A device isolation layer 56 is formed in the semiconductor substrate 50to define a plurality of active regions 58. The device isolation layer56 is formed in the first well region 54 of a first conductivity type. Aground select line GSL, a string select line SSL, and word lines WLn areformed such that they cross over the active regions 58.

The ground select line GSL and the string select line SSL may include aselect gate 62 s formed on a gate insulating layer 60, and a maskpattern 64 s may be formed on the select gate 62 s. The word line WLnmay include a gate electrode 62 w formed on the gate insulating layer 60and a mask pattern 64 w formed on the gate electrode 62 w. The gateinsulating layer 60 may be a tunnel insulating layer in a floating gatetype nonvolatile memory device, and may be a multi-layered charge traplayer in a charge trap type nonvolatile memory device.

An interlayer insulating layer 66 is formed on the semiconductorsubstrate 50. A semiconductor layer is formed on the interlayerinsulating layer 66. The semiconductor layer may be formed by bonding asemiconductor substrate or using crystal growth. A semiconductor plug 68is formed so as to penetrate the interlayer insulating layer 66 toprovide a seed layer for crystal growth. The semiconductor plug 68 maybe an epitaxial layer crystal grown from a predetermined region of thesemiconductor substrate 50. The epitaxial layer may be grown from asource region 55 s and/or a drain region 55 d of a cell array.

Referring to FIGS. 5A and 5B, a first semiconductor layer 70 is formedthrough crystal growth from the semiconductor plug 68. The firstsemiconductor layer 70 may be an epitaxial layer epitaxially grown fromthe semiconductor plug 68 using chemical vapor deposition (CVD).Alternatively, the first semiconductor layer 70 may be formed by formingamorphous silicon and then crystallizing the amorphous silicon usingannealing or laser. The first semiconductor layer 70 may be formed to athickness of approximately 1000 Å. The first semiconductor layer 70 maybe formed at 800° C. or lower to suppress the property degradation ofunit devices formed thereunder.

Impurities 72 are implanted into the first semiconductor layer 70 toform a second well region of a first conductivity type having a dopingconcentration higher than that of the first well region 54. Theimpurities 72 may be p-type impurities, and a dose of the impurities 72greater than an impurity dose of the first well region 54 may beimplanted. The first semiconductor layer 70 may be the second wellregion of a first conductivity type.

Referring to FIGS. 6A and 6B, a second semiconductor layer 74 is formedon the first semiconductor layer 70. The second semiconductor layer 74may be crystal grown using the first semiconductor layer 70 as a seedlayer. The second semiconductor layer 74 may be formed thicker than thefirst semiconductor layer 70. Impurities 76 are implanted into thesecond semiconductor layer 74 to form a third well region of a firstconductivity type. The third well region may have a doping concentrationless than that of the second well region, and has a doping concentrationequal or similar to that of the first well region 54. The secondsemiconductor layer 74 may be the third well region. Therefore, a dopingconcentration of the first semiconductor layer 70 may be greater thanthat of the second semiconductor layer 74.

In some embodiments of the present invention, the second and third wellregions may be formed by in situ doping, not using impurityimplantation. For example, the second and third well regions may bedoped by implanting impurities while the first and second semiconductorlayers 70 and 74 are crystal grown.

Referring to FIGS. 7A and 7B, the second semiconductor layer 74 isetched to form a trench region, and a device isolation layer 78 isformed such that it fills the trench region. The device isolation layer78 may be formed only in the second semiconductor layer 74, or mayvertically extend to the first semiconductor layer 70. Therefore, thethird well region may be limitedly formed between the device isolationlayers 78 and its border may be defined by the device isolation layer78.

Next, a gate insulating layer 80 is formed on the second semiconductorlayer 74, and a ground select line, a string select line, and word linesare formed on the gate insulating layer 80 to form the structureillustrated in FIGS. 3A and 3B. Although not shown, a plurality of cellarray regions may be formed by repeatedly forming an interlayerinsulating layer and a semiconductor layer on the second semiconductorlayer 74.

To form the device isolation layer 78 in the second semiconductor layer74, the semiconductor layer 74 may have a sufficient thickness fordevice isolation. Although not shown, wiring is connected to thesemiconductor substrate 50 so as to penetrate the semiconductor layer.When the semiconductor layer is thick, the etching burden may increase.Therefore, the semiconductor layer that the wiring penetrates may bethin. In this respect, a method may be provided of forming the secondsemiconductor layer 74 only in a region where the wiring is not formed,which will be fully described below.

FIGS. 8A through 10A are sectional views taken along the line I-I′ ofFIG. 2 for illustrating a nonvolatile memory device and a method offabricating the same, according to further embodiments of the presentinvention. FIGS. 8B through 10B are sectional views taken along the lineII-II′ of FIG. 2 for illustrating a nonvolatile memory device and amethod of fabricating the same, according to further embodiments of thepresent invention.

Referring to FIGS. 8A and 8B, similar to the embodiments discussedabove, a first semiconductor layer 70 is formed where a second wellregion of a first conductivity type is formed. A mold pattern 71 havingopenings 73 is formed to define an active region. The mold pattern 71may correspond to a device isolation region. Therefore, the mold pattern71 may be formed to a sufficient thickness for device isolation. Also,the mold pattern 71 may be formed in a region where wiring connected tothe semiconductor substrate 50 is formed.

Referring to FIGS. 9A and 9B, a semiconductor layer may be formed on anentire surface of the semiconductor substrate 50 to form a secondsemiconductor layer 174 so as to fill the openings 73. The secondsemiconductor layer 174 may be crystal-grown from the firstsemiconductor layer 70 exposed through the openings 73 to fill theopenings 73. Next, impurities are implanted into the secondsemiconductor layer 174 to form a third well region of a firstconductivity type. A border of the third well region may be defined bythe mold pattern 71. The third well region may have a dopingconcentration less than that of the second well region, and have a wellconcentration equal or similar to the first well region.

Referring to FIGS. 10A and 10B, a gate insulating layer 180 is formed onthe second semiconductor layer 174, and a ground select line GSL, astring select line SSL, and word lines WLn are formed on the gateinsulating layer 180. The mold pattern 71 may correspond to a deviceisolation layer, and a bottom surface of the device isolation layer maycontact an upper surface of the first semiconductor layer 70.Identically to FIG. 3A, a source region 75 s and a drain region 75 d maybe formed in the second semiconductor layer 74.

In the above described embodiments of the present invention, the secondwell region may be limitedly formed in the first semiconductor layer 70such that it is aligned with the active region defined in the secondsemiconductor layer 74 and 174 by implanting impurities into the firstsemiconductor layer 70 using the device isolation layer 78 or the moldpattern 71 as an ion implantation mask.

FIG. 11 is a cross-sectional view for illustrating a well structureaccording to some embodiments of the present invention.

Referring to FIG. 11, the same well structure as that formed on thesemiconductor substrate 50 may be formed on the first semiconductorlayer 70. That is, the first semiconductor layer 70 may include a secondconductivity type well 70 b and a second well region 70 t of a firstconductivity type formed on the second conductivity type well 70 b. Thefirst semiconductor layer 70 may include a plurality of second wellregions 70 t of a first conductivity type. The second conductivity typewell 70 b may be formed under the second well regions 70 t or betweenthe second well regions 70 t.

Some embodiments of the present invention may be applied to a floatinggate type nonvolatile memory device and a charge trap type nonvolatilememory device.

FIGS. 12 and 13 are cross-sectional views illustrating structures of anonvolatile memory device, according to some embodiments of the presentinvention.

Referring to FIG. 12, gate insulating layers of the charge trap typenonvolatile memory device include a stack structure of tunnel insulatinglayers 60 a and 80 a, charge trap layers 60 b and 80 b, and blockinginsulating layers 60 c and 80 c, respectively. Memory cell gateelectrodes 162 w and 182 w may be formed on the gate insulating layersto constitute word lines WL, and select gate electrodes 162 s and 182 sare formed on the gate insulating layers to constitute select gate linesSL, respectively.

On the other hand, a gate insulating layer 60 of the floating gate typenonvolatile memory device may be a single layer. A gate insulating layerof the word line WL may have a thickness equal to or less than a gateinsulating layer of the select gate lines SL.

The word line WLn may include a floating gate 362 w, an inter-gatedielectric layer 63 w, and a control gate electrode 462 w. The selectline SL may include a lower gate 362 s, an inter-gate insulating layer63 s, and an upper gate 462 s. A portion of the inter-gate insulatinglayer 63 s may be removed to connect the lower gate 362 s to the uppergate 462 s.

In some embodiments of the present invention, the semiconductor layerformed over the semiconductor substrate may be formed by using epitaxialgrowth using CVD, crystallization of amorphous silicon using annealingor laser after forming the amorphous silicon, and/or bonding asemiconductor substrate onto an interlayer insulating layer. Thesemiconductor layer may be thinner than the well formed in thesemiconductor substrate and the first semiconductor layer may be thinnerthan the second semiconductor layer. For example, the depth of the wellregion formed in the semiconductor substrate may be approximately 6000Å, the thickness of the first semiconductor layer may be approximately1000 Å, and the thickness of the second semiconductor layer may beapproximately 2000 Å. Also, the semiconductor layer may be formed at atemperature of about 800° C.

After forming the semiconductor layer and/or after forming the deviceisolation layer in the semiconductor layer, semiconductor crystals maybe hardened or their crystallization may be improved using thermal orlaser annealing.

As described above, according to some embodiments of the presentinvention, because electrical resistances are equal or similar to eachother between a semiconductor substrate and a semiconductor layer wherecell arrays are formed, a voltage drop associated with the semiconductorlayer can be suppressed and sufficient current can be supplied.

Also, although the semiconductor layer is not relatively thick, a wellresistance can be reduced by increasing a well concentration, and thesemiconductor layer can be formed in the same well structure as that ofthe semiconductor substrate, thereby decreasing dispersion of cellproperty of a memory device.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe true spirit and scope of the present invention. Thus, to the maximumextent allowed by law, the scope of the present invention is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

1. A method of fabricating a memory device, the method comprising:forming a first well region of a first conductivity type in asemiconductor substrate; forming an interlayer insulating layer on thesemiconductor substrate; forming a first semiconductor layer on theinterlayer insulating layer; forming a second well region of the firstconductivity type having a doping concentration greater than a dopingconcentration of the first well region in the semiconductor layer;forming a second semiconductor layer on the first semiconductor layer;and forming a third well region of the first conductivity type having adoping concentration equal to or less than a doping concentration of thesecond well region in the second semiconductor layer; wherein the dopingconcentration of the third well region is approximately equal to thedoping concentration of the first well region.
 2. The method of claim 1,wherein a thickness of the first semiconductor layer is less than athickness of the first second semiconductor layer.
 3. The method ofclaim 1, wherein forming of the first semiconductor layer comprises:forming a second conductivity type well on the interlayer insulatinglayer; and forming the second well region on the second conductivitytype well.
 4. The method of claim 3, wherein a plurality of the secondwell regions are formed in the first semiconductor layer, the secondwell regions are separated from each other by the second conductivitytype well.
 5. The method of claim 4, wherein a plurality of second wellregions are formed in the first semiconductor layer and are isolatedfrom each other by the second conductivity type well region.
 6. Themethod of claim 1, wherein forming the second semiconductor layercomprises forming a mold pattern having an opening to define an activeregion in the first semiconductor layer; wherein the secondsemiconductor layer substantially fills the opening.
 7. The method ofclaim 1, further comprising: patterning the second semiconductor layerto form a trench region defining an active region; and filling thetrench region with an insulating layer to form a device isolation layer.8. The method of claim 7, wherein a bottom surface of the deviceisolation layer contacts a top surface of the first semiconductor layer.9. The method of claim 7, wherein the second well region is aligned withthe device isolation layer and formed in the first semiconductor layerunder the active region.
 10. The method of claim 1, wherein forming ofthe first semiconductor layer comprises; forming a semiconductor plugthat penetrates the interlayer insulating layer; and forming the firstsemiconductor layer through crystal growth from the semiconductor plug.11. The method of claim 1, wherein a depth of the first well region isgreater than a depth of the second well region.
 12. A method offabricating a memory device, the method comprising: providing asemiconductor substrate having a first well region of a firstconductivity type; forming a first device isolation layer on thesemiconductor substrate to define a plurality of first active regions;forming an interlayer insulating layer on the semiconductor substrate;forming a semiconductor plug that penetrates the interlayer insulatinglayer; forming the semiconductor layer having a second well region ofthe first conductivity type on the interlayer insulating layer throughcrystal growth from the semiconductor plug; and forming a second deviceisolation layer on the at least one semiconductor layer to define aplurality of second active regions; wherein a doping concentration ofthe second well region is greater than a doping concentration of thefirst well region.
 13. The method of claim 12, wherein forming of thesemiconductor layer comprises: forming a first semiconductor layerhaving the second well region on the interlayer insulating layer; andforming a second semiconductor layer having a third well region of thefirst conductivity type of which a doping concentration is equal to orless than the doping concentration of the second well region on thefirst semiconductor layer; wherein a doping concentration of the thirdwell region is approximately equal to a doping concentration of thefirst well region.
 14. The method of claim 13, wherein the third wellregion is formed between the second device isolation layers in thesecond semiconductor layer and a border of the third well region isdefined by the second device isolation layers.
 15. The method of claim12, further comprising; forming a second conductivity type well regionunder the second well region in the first semiconductor layer.
 16. Themethod of claim 15, wherein a plurality of second well regions areformed in the first semiconductor layer, and the second conductivitytype well is formed under the second well regions.
 17. The method ofclaim 12, further comprising: forming a first cell array on thesemiconductor substrate: and forming a second cell array on thesemiconductor layer.
 18. The method of claim 17, wherein the first andsecond cell arrays comprises NAND type non-volatile memory cells.